Field of the Invention
The present invention relates to polymeric foam containing a polyhedral oligomeric silsesquioxane grafted polymer and a process for preparing such foam.
Introduction
Polymeric foam is a commonly used thermally insulating material. Historically, typical thermally insulating polymeric foam has an average cell size of 100-200 micrometers. However, reducing the average cell size of polymeric foam to the nanometer scale results in a material having greater thermal insulating properties. For example, reducing the average cell size from one micrometer to 300 nanometers (nm) reduces nearly in half the cell gas contribution to thermal conductivity through a foam while reducing the average cell size from one micrometer to less than 100 nm reduces that value to ⅓ of the one micrometer value. Therefore, polymeric foam having nanometer-sized cells (that is, having an average cell size that is less than one micrometer) is desirable as thermally insulating material.
Preparing polymeric foam having nanometer-sized cells at a high enough porosity to effectively serve as thermally insulating material is challenging. Highly porous foam is desirable to reduce thermal conductivity component through the polymer matrix of foam. Foam that is highly porous has more void volume and less polymer volume in the foam structure. Desirably, polymeric foam has a porosity of 70% or higher to minimize the thermal conductivity through the polymer. One method for preparing polymeric foam having nanometer-sized cells (that is, nanofoam) that has a high enough porosity to serve as a thermally insulating material uses a nanometer-sized nucleating additive during the foaming process. For example, WO2011/066060 discloses preparation of nanofoam by incorporating nano-sized nucleating additives such as polyhedral oligomeric silsesquioxane (POSS). It is difficult to disperse nano-sized nucleating additives into a polymer composition efficiently so that they can effectively serve as nucleating agents. WO2011/066060 discloses a process of providing and introducing into a polymer the nano-sized nucleating additives in a sol, wet gel, slurry or solution. Such a method requires a carrier medium distinct from the nano-sized nucleating additive.
U.S. Pat. No. 6,933,345 discloses grafting POSS to polymers to produce compatibilization aids for use in compounding, blending and alloying of polymers with non-reactive POSS reinforcements and other nanoscopic particulates as well as to control polymer morphology. POSS-grafted polymers offer a means for compatibilizing POSS with other materials. However, U.S. Pat. No. 6,933,345 offers only a means for creating polymer blends with the POSS-grafted polymers. Polymers grafted with POSS are larger than nano-sized particles and, while U.S. Pat. No. 6,933,345 teaches that they are useful compatibilization aids, it remains unclear if they are of any value as nucleating agents for creating nano-sized cells in polymeric foam.
It is desirable to find a way to introduce nano-sized nucleating additives into a polymer without using a distinct carrier medium, yet in a manner that allows the nano-sized nucleating additives to nucleate sufficiently to form polymeric foam having nano-sized cells, particularly such polymeric foam having a porosity high enough to serve as a thermally insulating material.